1. Field of Invention
This invention relates to the field of x-ray spectrometry and other instrumentation relying on energy dispersive spectrometry analysis of x-rays emitted from a target, and specifically to a method and apparatus for preserving the flux of x-rays emitted from said target and available for analytical purposes within the instrument.
2. Description of Prior Art
In x-ray spectroscopic elemental analyses, there are two main techniques for separating into their various energies the x-rays emitted from the sample under analysis, these are wavelength dispersive spectroscopy (WDS) and energy dispersive spectroscopy (EDS).
WDS uses Bragg reflection from a crystal to separate the x-rays into various wavelengths, while EDS uses a solid state device whose output is a known function of the x-ray energy to separate them into various energy bins. For reasons of mechanical simplicity, data collection times, spectrometer size and various other technical reasons, EDS is frequently preferred over WDS. Unfortunately, EDS systems are frequently inefficient at detection of very low energy x-rays and this inefficiency is exacerbated by the low production rate of low energy x-rays compared to higher energy ones. These low energy x-rays correspond to those emitted by light elements such as Be, B, C, N, O, and F and also the so-called L lines of the heavier elements. Because of the low production rate of these low energy x-rays compared to higher energy ones, an EDS system spends most of its time counting pulses from higher energy x-rays, thus adversely affecting the instrument""s detection limit for the light elements, thereby making these elements detectable by EDS only when found in relatively high concentration in the sample. The x-ray lines for the light elements are closer in energy than those for the heavier elements and many EDS systems have difficulty resolving these x-ray lines when L lines from heavier elements are also emitted by the sample. EDS cannot be used for light element analyses except in special cases or if the EDS detector is specially configured for optimum resolution. Resolution is often a decreasing function of detector size, because a smaller detector has less capacitance than a larger one. Also, a small detector subtends a smaller collection angle than a larger one, resulting in a lower count rate. In some cases, the size of the detector is limited by the instrument""s geometry, so that a smaller detector is necessary even when poor resolution occurs. For example, some electron microscopes have sufficiently close working distances or otherwise poor access to x-rays emitted from a sample, that either a very small detector must be used or the detector must be placed far from the x-ray source.
Accordingly, several objects and advantages of the present invention are:
a) to solve the practical problem of increasing the efficiency of x-ray detection in an energy dispersive x-ray spectrometer by gathering x-rays that are diverging away and redirecting them through an aperture;
b) to provide an energy dispersive x-ray spectrometer which collects data faster than existing systems;
c) to provide an energy dispersive x-ray spectrometer which is more sensitive than previous instruments,
d) to teach a method to limit the area of the target from which x-rays reach the detector, thereby allowing the analysis of very small areas within a sample.